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Unformatted text preview: ECE ECE 3534 Digital Design 1
Section 5a: Review of Flip-Flops Flip Flip-Flops
• We can describe flip-flops in terms We of their characteristic information. • Characteristic information describes the next state of the flipflop as a function of the present state and all of the flip-flop inputs. • Instead of considering the operation of the flip-flop in terms of all of the distinct combinations ll di of input and present state, we can describe the flip-flop in terms of the functions that each combination of the inputs carries out. The The SR Flip-Flop
S 0 0 1 1 R 0 1 0 1 Q Q+ 0 1 0 1 0 1 0 1 Function The characteristic equation for the SR flip flip-flop is Q+ = S + R′Q, but this but characteristic equation only applies for valid values of S and R. The The D Flip-Flop
Imagine Imagine that an SR flip flop has had its S and R inputs connected through an inverter. Call the new input D.
S 0 1 R 1 0 D 0 1 Q Q+ 0 1 0 1 Function The characteristic equation of the D flip-flop is Q+ = D. The The JK Flip-Flop
Consider Consider the JK flip-flop as an improved SR flip-flop. Unlike the SR flip-flop, the input J = K = 1 has a valid function. lid
J 0 0 1 1 K 0 1 0 1 Q Q+ 0 1 0 1 0 1 0 1 Function The characteristic equation for the JK JK flip-flop is Q+ = JQ′ + K′Q. The The T Flip-Flop
Imagine Imagine that a JK flip flop has had its inputs connected. Call the new input T.
J 0 1 K 0 1 T 0 1 Q Q+ 0 1 0 1 Function The characteristic equation for the T flip-flop is Q+ = T ⊕ Q. Flip Flip-Flops
• Characteristic information answers Characteristic the question “Based on the present information, what will the next state be?” • This information is important, but it doesn’t answer an important design question. Generally, we know what the present state is and what the next state should be. • Our question is “Given a known present state, and a desired next state, what do the flip-flop inputs have to be to achieve the state transition? transition?” Excitation Excitation Information
• This information is called excitation This information. • We can derive the excitation information for each flip-flop by reorganizing its characteristic information. • If we consider the transitions as functions, we can determine which functions of the flip-flop achieve fli each transition. Excitation Excitation Information
SR flip-flop • Functions: no change, set, reset Function? Q 0 0 1 1 Q+ 0 1 0 1 S R Excitation Excitation Information
D flip-flop • Functions: set, reset Function? Q 0 0 1 1 Q+ 0 1 0 1 D Excitation Excitation Information
JK flip-flop • Functions: no change, set, reset, toggle
Function? Q 0 0 1 1 Q+ 0 1 0 1 J K Excitation Excitation Information
T flip-flop • Functions: no change, toggle Function? Q 0 0 1 1 Q+ 0 1 0 1 T ...
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